The USPTO maintains two trademark registers, the Principal Register and the Supplemental Register. The Principal Register is the place for the traditional, good against the world, marks like Coke and Pepsi. A critical requirement of registration on the Principal Register is uniqueness, either initially or established over time.
The Supplemental Register is for those trademarks which are principally descriptive and won’t qualify for initial registration on the Principal Register. The marks must be capable of distinguishing goods and services (even though they may not actually serve that purpose). An example might be The Brewery, for a business that brews beer. A mark that has been on the Supplemental Register for 5 years has acquired “secondary meaning” and can be transferred to the Principal Register.
Certainly, if a mark can go on the Principal Register, that is the place to go. Many more rights flow from such a registration. Nevertheless, registration on the Supplemental Register is beneficial in that it permits use of the ®, the symbol of federal registration and serves as a deterrent to junior users of the mark.
Caveat: Any mark, whether on the Principal or Supplemental Register, is potentially subject to attack by a superior, common law user.